Add new component label to relevant issuses

[xuanshenbo]

As an Angular Material user, I want to see a list of all in-progress and planned new components, so that I don't submit duplicate new component requests.

Currently I'm aware of three places where we can see available, in-progress and planned components:

1. Doc site (available components only)
2. README.MD
3. Issues page with new component label

However, neither README.MD nor Issues page with new component label is a complete list of all in-progress and planned components. For example, README.MD does not have #2691, and neither #2691 nor #508 has a new component label.

I reckon the easiest way would be adding a new component to all new components that have an open issue already.

This may reduce the number of duplicate new component requests.

[jelbourn]

The README should capture everything we plan on doing at a high level; if something is missing from the README, it's not a total lock-in for something we're definitely going to have.

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